The present invention relates to a protective cover for a solar cell, and, more particularly, to an output-increasing, protective cover for a solar cell.
Various types of photovoltaic ("PV") devices such as solar cells, for converting radiant energy, such as sunlight, into electricity are known. One type of solar cell which is of particular current interest comprises a plurality of spaced particles or members, typically spherical or spheroidal particles, supported by a conductive sandwich which includes first and second conductive sheets separated by an insulative coating. Each sphere is a semiconductor, for example silicon having a P-type interior and an N-type exterior or skin.
The first sheet is a thin, flexible metallic foil, typically aluminum, with a plurality of spaced cell-retaining apertures formed therethrough, for example, by an emboss-then-etch or stamping process. The apertures may define a regular geometrical pattern which preferably comprises overlapping hexagons. This pattern permits maximum packing of the apertures, and, hence, of the spheres. The spheres are placed on a top surface of the first foil and, by the use of negative pressure and doctoring techniques, each aperture ultimately has one sphere nested therein. Thereafter, heat and pressure are applied to the cell sandwich to move the nested spheres partially into and through the apertures. This movement effects the interaction of the aperture walls with the spheres to locally remove the native aluminum and silicon oxides so that the abutting aluminum mechanically bonds directly to, and forms an electrical contact with, the N-type exterior of the silicon spheres, thereby affixing the spheres to the first foil.
The affixing of the spheres to the first foil results in an upper light-gathering portion of each sphere protruding or extending above the top surface of the first foil and a lower portion of each sphere protruding below a lower surface of the first foil. The N-type exterior is removed from the cells below the first foil's lower surface. The lower foil surface and the exposed P-type interior of the lower sphere portions are then coated with a flexible, electrically insulative coating, typically a polyamide. The insulative coating on the spheres is then treated to remove some of the coating to thereby expose the P-type interior of each sphere through so-called vias. Following via formation, the second foil, preferably flexible aluminum, is electrically connected to the P-type interiors of the spheres through the vias. The flexible solar cell so formed may power utilization devices connected between the foils.
The foregoing and similar solar cells and techniques for producing them are disclosed in the following commonly assigned U.S. Pat. Nos.: 4,407,320; 4,521,640; 4,581,103; 4,582,588; 4,806,495; 4,872,607; 4,917,752; 4,957,601; 5,028,546; 5,192,400; 5,091,319; and 5,086,003.
The above-described solar cells comprise a plurality of miniature PV devices--the spaced silicon or other semiconductive members, spheres, spheroids or other particles--connected in electrical parallel via the first and second foils. The foils, therefore, are connectable to a utilization device or circuit for electrical energization thereof when the cells are exposed to radiant energy. The cells are flexible and may be formed into various non-planar configurations, either free-standing or conforming to an irregular underlying surface.
While solar cells constructed as set forth above are mechanically robust, protecting them from the deleterious effects of the environment and ambient conditions is generally thought to be prudent. For example, water in the form of rain or other precipitation, in prolonged direct contact with the spheres or other particles or the foils can cause oxidation and corrosion, giving rise to mechanical and/or electrical degradation of the cell. Pollutants may also deleteriously affect the cell, such as by attacking the spheres or foil of the cell or by decreasing or preventing radiant energy from reaching the particles or spheres.
For the foregoing and other reasons, it is typical to cover, encapsulate or otherwise house solar cells to protect them against ambient-caused degradation. Such protective measures viewed in the context of prior art non-flexible solar cells have often taken the form of rigid "picture frames" having a transparent cover which surrounds the solar cell to isolate it from the ambient. The cover, of course, permits sunlight and other radiation to reach the cells where it is converted to electricity. Such picture fame covers are not flexible and limit the range of uses to which the flexible cells of the above patents may be put.
The upper portion of each particle or sphere--typically an N-type silicon hemisphere--functions as a spherical lens. That is, this upper portion gathers light incident on the particle or sphere and directs this light onto the particle's or sphere's P-N junction. These spherical lenses are able to direct to the P-N junction only that light which is directly incident on the particles or spheres. Some of the light which is incident on the top surface of the first foil between the particles or spheres--that is, light which "misses" the particles or spheres--is, in effect, "wasted" and is not effective to produce electricity, because it does not reach the P-N junction of the spheres, and is, instead, back-reflected to the ambient.
Commonly assigned U.S. patent application, Ser. No. 08/060,773, filed May 11, 1993 (now U.S. Pat. No. 5,419,782) discloses a flexible protective cover with an undulating free surface which comprises a coating having particle-conformal portions which extend into the spaces between adjacent particles. The portions of the coating conforming to each particle act as lenses, directing otherwise "wasted" light--light which would fall on the first foil between adjacent particles for reflection back along its incoming path--onto the underlying particle. While this cover achieves solar cell efficiency increases of about 10%-20% (for a given amount of radiant energy incident on the cell), dirt and pollution-borne contaminants can be difficult to remove from its undulating irregular free surface.
Commonly assigned U.S. patent application, Ser. No. 08/212,542, filed Mar. 14, 1994 discloses another cover for a similar photovoltaic cell. The cover includes a layer of radiant-energy transparent material having a free or upper surface and an opposed surface. The opposed surface is irregular. This irregularity results from the opposed surface conforming to the polar or upper regions of the particle portions and being configured into prism-like cusps, extensions, projections or deformations between adjacent particles. The cusps approach, but do not contact the reflective surface. The resulting spaces, gaps or volumes between the cusps and the reflective surface may contain a substance, such as air, other gases, a liquid, a polymer of a different index of refraction, or may be evacuated. The material between adjacent particles--that is, the bottoms of the cusps--may be concave or convex as viewed from the reflective surface.
The configuration of the cusps and the refractive indices of the layer and the resulting spaces are related and cooperate so that a significant amount of the radiant energy which passes through the layer between adjacent particles--which radiant energy, as noted above, would be otherwise "wasted"--is refracted at the cusp-space interface and is then reflected by the reflective surface onto the particle portions. Specifically, refraction causes the radiant energy to impinge on and to be reflected from the reflective surface in a non-perpendicular manner. The non-perpendicularity of the reflection directs the energy onto the particle portions. The cover may increase cell efficiency up to 25% as it protects the cell from harm caused by the ambient.